Field Of The Invention
The present invention relates to a ceramic rotatable magnetron sputtering cathode target to be used for forming a ceramic film, particularly a transparent non-crystalline oxide film having excellent durability or a transparent oxide film having a low refractive index, by sputtering, and a process for its production.
Discussion Of Background
Heretofore, it has been known to impart an optical function by forming a thin film on a transparent substrate such as glass or plastic, for example, an anti-reflective coating on a mirror, a heat radiation reflecting glass, a low emissivity glass, an interference filter, a camera lens or an eye glass.
In the case of a usual mirror, a film of Ag is formed by an electroless plating method, or a film of Al or Cr is formed by a vacuum deposition method or a sputtering method. Among such films, a Cr film is relatively tough and is used as a surface-coated mirror with the coating surface exposed.
In the case of a heat radiation reflecting glass, it has been common to form a film of titanium oxide or tin oxide by a spraying method, a CVD (chemical vapor deposition) method or a dipping method. Recently, it has become common to employ a heat radiation reflecting glass having a metal film, a nitride film or a tin-doped indium oxide (ITO) film formed on a glass surface by a sputtering method. With the sputtering method, the film thickness can easily be controlled, and a plurality of films can continuously be formed, whereby it is possible to design e.g. the transmittance, the reflectance and the color by a combination with transparent oxide films. Therefore, the demand is increasing in such a field as building construction where a design or appearance is of importance.
A low emissivity glass which reflects a radiant heat from a wall or an air conditioner in a room to the inside of the room, usually has a structure having a silver layer sandwiched between zinc oxide layers such as a three-layered system of ZnO/Ag/ZnO or a five-layered system of ZnO/Ag/ZnO/Ag/ZnO, and thus is used in the form of a multi-layered glass or laminated glass. In recent years, this glass has found a remarkable spread of use in cold areas in Europe.
The anti-reflective coating for lenses, etc. is usually formed by alternately laminating a high refractive index film such as titanium oxide or zirconium oxide and a low refractive index film such as silicon oxide or magnesium fluoride. Usually, a vacuum deposition method is employed, and the substrate is heated during the film forming operation to improve the scratch resistance.
The surface-coated mirror, the heat radiation reflecting glass of a single plate form or the anti-reflective coating for lenses, etc., is used under such a condition that the coated film is exposed to the air. Therefore, it is required to be excellent in the chemical stability and the abrasion resistance. On the other hand, the low emissivity glass is also likely to be damaged, for example, by scratching during the transportation or handling prior to being formed into a multi-layered glass or laminated glass. Therefore, a protecting film or an optical thin film serving also as a protecting film, which is excellent in the stability and the abrasion resistance, is desired.
To improve the durability, it has been common to provide a chemically stable transparent oxide film on the air side. As such an oxide film, a film of titanium oxide, tin oxide, tantalum oxide, zirconium oxide or silicon oxide may be mentioned, and such an oxide film has been selected for use depending upon the required properties.
Titanium oxide and zirconium oxide are excellent in the chemical stability, but they tend to form crystalline films and the surface roughness tends to be substantial, whereby the friction will be large when they are abraded, and they are inferior in the abrasion resistance. On the other hand, tin oxide and silicon oxide are, respectively, poor in the acid and alkali resistance, and they are not durable when immersed in an acidic or alkaline solution for a long period of time. Among them, tantalum oxide has both of the abrasion resistance and the chemical stability, but its abrasion resistance is not fully satisfactory.
Further, titanium oxide, tin oxide, tantalum oxide and zirconium oxide have relatively high refractive indices, whereas silicon oxide has a relatively low refractive index. Thus, there is a limitation in the degree of freedom for optical designs to impart various optical functions.
It is therefore desired to develop a thin film having a high level of durability and a high degree of freedom for a wide range of optical designs. In Japanese Patent Application No. 47133/1990, the present applicants have proposed, as such a highly durable thin film, a target comprising, as the main components, at least one member selected from the group consisting of Zr, Ti, Hf, Sn, Ta, In and Cr and at least one member selected from the group consisting of B (boron), Si (silicon) and O (oxygen), and a non-crystalline oxide film formed by sputtering the target.
On the other hand, with respect to a transparent thin film having a low refractive index, it has been common to provide a chemically stable transparent oxide film on the air side in order to improve the durability, as mentioned above. As such an oxide film, a film of titanium oxide, tin oxide, tantalum oxide, zirconium oxide or silicon oxide may, for example, be mentioned, and as a typical film having a low refractive index, magnesium fluoride may, for example, be mentioned. Such an oxide film has been selected for use depending upon the desired properties. Titanium oxide, tin oxide, tantalum oxide and zirconium oxide have high refractive indices, whereas silicon oxide and magnesium fluoride have low refractive indices.
However, such a film can hardly be formed on a substrate having a large area and can not be applied to an object where formation of a film covering a large area is required, such as glass for buildings or glass for automobiles. A DC (direct current) sputtering method is most suitable for forming a film with a large area. However, there has been no target material suitable for providing a transparent thin film having a low refractive index. Thus, it has been impossible to obtain a desired thin film by means of a DC sputtering method which is capable of forming a film with a large area.
For example, in order to form a silicon dioxide thin film by a DC sputtering method, it is conceivable to employ a method wherein an electrically conductive Si target is subjected to reactive sputtering in an oxygen-containing atmosphere to from a silicon dioxide thin film. However, the surface of such Si target is oxidized during the sputtering, whereby the conductivity decreases, and sputtering can hardly be continued under a stabilized condition. Further, the formed silicon dioxide thin film has poor durability against alkalinity and is not durable when dipped in an alkaline solution for a long period of time.
Under these circumstances, the present applicants have proposed in Japanese Patent Application No. 201149/1990 a non-crystalline oxide thin film and a target which has high durability and a low refractive index as well as a high degree of freedom for a wide range of optical designs.
On the other hand, a planar target for sputtering is prepared through a long series of process steps including mixing of inorganic compounds as raw materials, followed by molding, calcination and processing to a shape suitable for the sputtering apparatus and bonding. In the molding, calcination, processing and bonding steps among them, a large scale apparatus is required for the preparation of a large size target for a production machine, although no such a large scale apparatus is required when the target is small in size. Further, in the bonding step, when a ceramic target is to be bonded on a target holder metal plate, it is divided for processing and bonding, whereby a large scale apparatus and a large amount of an expensive indium solder are required, and thus a substantial labor force and costs are required.
Further, in sputtering of a glass sheet with large area for buildings, a high sputtering power is applied to increase the film-forming speed in order to increase the productivity. However, cooling of the target limits the film-forming speed and causes troubles such as peeling and cracking of the target. A rotatable magnetron sputtering cathode target of new type having such aspects improved, is known. This is of a type where a magnetic field-generating means is disposed inside a cylindrical target, and sputtering is conducted while rotating the target and cooling the target from inside, whereby a power larger per unit area than the planar target, can be applied, and accordingly film-forming can be conducted at a high speed. In most cases, such a target is a cylindrical rotatable cathode made of a metal or alloy to be sputtered, and when the substance to be sputtered is a soft or brittle metal or alloy, such a substance is formed on a cylindrical target holder.
In the case of a metal target, a multi-layered film coating of e.g. the oxide, nitride and carbide, is possible using various sputtering atmospheres, but it has drawbacks such that the coating film is likely to be damaged by different types of atmospheres so that the desired composition can not be obtained and in the case of a low melting point metal target, it is likely to be melted when a power is applied too much. Therefore, a ceramic target has been desired.
Further, a method of preparing a sputtering target by thermal spraying, has been proposed. However, this method has problems such that the difference in the thermal expansion between the ceramic and the metal is so large that the spray coating film can not be made thick, and that due to the thermal shock during the use, the adhesion of the film tends to deteriorate, thus resulting in its peeling. Further, there is a method wherein a sintered body of ceramic is prepared in the form of a cylinder, and it is bonded to a target holder material by means of indium metal. However, this method is cumbersome and costly.